This is a crossover intervention study, where men with prostate cancer (n = 17) were fed rye whole-grain/bran or refined wheat products for 6-wk periods. The results show that a single fasting plasma sample could be used to estimate the mean plasma alkylresorcinols concentration at a precision of +/- 20%. This suggests that the plasma Alkylresorcinols concentration can be used as a reliable biomarker for whole-grain wheat and rye under intervention conditions where intake is kept constant.